The present invention relates to an acceleration sensor and a method of producing the same. More specifically, the present invention relates to a semiconductor acceleration sensor for detecting three-dimensional acceleration.
FIGS. 8(a) and 8(b) are views showing an example of a conventional acceleration sensor 210 for detecting three-dimensional acceleration. The acceleration sensor 210 includes a weight 230; a base portion 220, and beams 240. The weight 230 is displaced upon receiving acceleration. The base portion 220 is disposed around the weight 230 and apart from the weight 230. The beams 240 are formed of thin wall members connecting the weight 230 and the base portion 220. More specifically, one end portions of the beams 240 are connected to the weight 230, and the other end portions of the beams 240 care connected to the base portion 220. A plurality of piezo resistance elements 250 is disposed on the beams 240 (refer to Patent Reference 1).
When acceleration is applied to the acceleration sensor 210, the weight 230 is displaced, so that the beams 240 deform accompanying with the displacement of the weight 230. When the beams 240 deform, a stress applied to the piezo resistance elements 250 changes, thereby changing resistance of the piezo resistance elements 250. In general, a change in the resistance of the piezo resistance elements 250 is very small. Accordingly, two pairs of the piezo resistance elements 250 (four elements) are formed in a bridge circuit on one of the beams 240, so that the change in the resistance of the piezo resistance elements 250 is detected as a change in a voltage.
In the acceleration sensor 210, it is possible to increase sensitivity thereof through increasing a mass of the weight 230; increasing a length of the beams 240; decreasing a width of the beams 240; or decreasing a thickness of the beams 240. However, when a width or a thickness of the beams 240 is decreased to increase sensitivity, strength of the beams 240 formed of silicon is lowered. Accordingly, when the acceleration sensor 210 receives large acceleration, the beams 240 may be easily broken.
In the conventional acceleration sensor 210, a silicon oxide layer is disposed under the weight 230 and apart from the weight 230 to obtain high sensitivity and high impact resistance at the same time. Further, stoppers 260 are disposed above the weight 230 for restricting a movement range of the weight 230.
In order to adjust sensitivity of an acceleration sensor, Patent Reference 2 discloses a configuration, in which a silicon nitride layer is formed on a beam, so that a layer thickness is adjusted.    Patent Reference 1: Japanese Patent Publication No. 2005-345294    Patent Reference 2: Japanese Patent Publication No. 2004-354074
In the conventional acceleration sensor described above, when the acceleration sensor disclosed in Patent Reference 1 is produced in a large quantity, it is difficult to obtain sufficient sensitivity and produce with a high yield due to poor impact resistance. That is, when a thickness of the beams is too large, it is difficult to obtain high sensitivity. On the other hand, when a thickness of the beams is too small, it is difficult to achieve high impact resistance, thereby reducing a yield. The sensitivity and the impact resistance have a trade-off relationship, and it is difficult to achieve high sensitivity and high impact resistance at the same time.
Even when a thickness of the beams is reduced to increase the sensitivity and a stopper is provided for improving the impact resistance, the thickness of the beams may vary during a production process. In this case, when the thickness of the beams is smaller than a specified level, the sensitivity is lowered, and when the thickness of the beams is larger than a specified level, the impact resistance is lowered.
Further, in producing the acceleration sensor, when the piezo resistance element is misplaced from a proper position, the sensitivity is lowered. More specifically, when the acceleration sensor is produced using a semiconductor laminate substrate, the piezo resistance element is produced through injecting ions from above the semiconductor laminate substrate. The proper position of the piezo resistance element is a location of the beams deforming most when the weight is displaced, i.e., a location close to the end portions of the beams. The semiconductor laminate substrate is etched from below to define locations of the end portions of the beams. Accordingly, depending on accuracy of the processes from above and below the semiconductor laminate substrate, the position of the piezo resistance element may vary.
In the acceleration sensor disclosed in Patent Reference 2, the silicon nitride layer is disposed on the beams, and the sensitivity is adjusted through changing the thickness of the silicon nitride layer. However, the acceleration sensor has the highest sensitivity when no silicon nitride layer is disposed on the beams. Accordingly, it is difficult to improve the sensitivity.
In view of the problems described above, an object of the present invention is to provide an acceleration sensor with improved sensitivity without lowering impact resistance. Further, an object of the present invention is to provide a method of producing the acceleration sensor.
Further objects and advantages of the invention will be apparent from the following description of the invention.